A 15-year-old boy has been charged by police after he allegedly filmed the assault in Bellevue Hill on March 4 and posted it online.

Detectives will question the boy's friend, also aged 15, over claims he sexually assaulted the 15-year-old girl at the party. Neither boy can be named for legal reasons.

The boy accused of assaulting the girl was a student at the elite Cranbrook School in Bellevue Hill and teachers at the school reportedly raised the alarm when they became aware of the allegations.

In an email sent on Tuesday afternoon, Cranbrook headmaster Nicholas Sampson told parents the school would offer support to students who needed it.

"You may be aware of a serious incident that is currently before the courts and has received media attention. This incident did not occur on school grounds," Mr Sampson wrote.

"As I am sure you will understand, we are not able to disclose further details at this stage, both because this is a police matter concerning a student under the age of 18 and because of our pastoral responsibilities for all our students.

"Should you feel your son requires support, please make contact with the school. We will continue, as always, to support our staff and students. This remains at the heart of what we do.

"As you know we, as a school, always place the highest priority upon the wellbeing of our students and students in other schools."

A Cranbrook School spokeswoman told Fairfax Media: "Our greatest priority is the safety and wellbeing of students."

"The matter is currently before the courts and as such it would be inappropriate for us to comment further at this stage," the spokeswoman said.

A NSW Police spokeswoman said an investigation into the alleged sexual assault was under way.

The Daily Telegraph reported that police would allege in court that the girl had been drinking alcohol and was unconscious when she was allegedly sexually assaulted and filmed. She did not know she had been assaulted until the footage emerged online days later, the newspaper reported.

The teenage boy charged with filming the assault appeared in Bidura Children's Court in Glebe on Monday.

The boy, who attends Rose Bay Secondary College, pleaded not guilty to charges of filming a person in a private act without consent, producing child abuse material, and disseminating child abuse material.